1. Field of the Invention
The present invention relates to an optical deflector which can swing a reflective plane.
2. Description of the Related Art
In the past, an optical deflector which includes a mirror part having a reflective plane and a pair of piezoelectric actuators actuating the mirror part was known (Patent Document 1). A waveform such as a sinusoidal waveform having periodicity is used for a voltage signal to be applied to the piezoelectric actuators of the optical deflector. In the optical deflector disclosed in Patent Document 1, voltage signals of a sawtooth waveform having periodicity and opposite phases are applied to the pair of piezoelectric actuators.
When such an optical deflector is used, for example, for an apparatus such as a projector which displays an image, unevenness in scanning speed causes deformation of an image in the scanning direction. Accordingly, only an area in which the scanning speed is constant is used to display an image. In order to keep the scanning speed constant, it is preferable that the voltage signals to be applied to the piezoelectric actuators be set to have a long straight (or substantially straight) section in one period.
A sawtooth waveform has a longer straight (or substantially straight) section in one period, compared with the sinusoidal waveform. Therefore, when the voltage signals to be applied to the piezoelectric actuators have a sawtooth waveform, the section in which the actuating speed of the minor part, that is, the scanning speed of the optical deflector, is constant can become longer in comparison with a case where the voltage signals have a sinusoidal waveform.
However, when the voltage signals have a sawtooth waveform, the optical deflector responds to the mechanical natural frequency relevant to the driving of the mirror part out of harmonic components included in the sawtooth waveform. Accordingly, a so-called ringing phenomenon occurs at the time of driving the mirror part and thus the scanning speed is not constant (speed uniformity degrades). Therefore, in the optical deflector disclosed in Patent Document 1, a natural frequency and the harmonic component of the natural frequency are removed from the harmonic components included in the sawtooth waveform.